Merge branch 'stable-3.2' into pandora-3.2

[pandora-kernel.git] / fs / eventpoll.c

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index 451b9b8..7903ddb 100644 (file)
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -477,8 +477,14 @@ static void ep_remove_wait_queue(struct eppoll_entry *pwq)
        wait_queue_head_t *whead;
 
        rcu_read_lock();
-       /* If it is cleared by POLLFREE, it should be rcu-safe */
-       whead = rcu_dereference(pwq->whead);
+       /*
+        * If it is cleared by POLLFREE, it should be rcu-safe.
+        * If we read NULL we need a barrier paired with
+        * smp_store_release() in ep_poll_callback(), otherwise
+        * we rely on whead->lock.
+        */
+       whead = ACCESS_ONCE(pwq->whead);
+       smp_mb();
        if (whead)
                remove_wait_queue(whead, &pwq->wait);
        rcu_read_unlock();
@@ -859,17 +865,6 @@ static int ep_poll_callback(wait_queue_t *wait, unsigned mode, int sync, void *k
        struct epitem *epi = ep_item_from_wait(wait);
        struct eventpoll *ep = epi->ep;
 
-       if ((unsigned long)key & POLLFREE) {
-               ep_pwq_from_wait(wait)->whead = NULL;
-               /*
-                * whead = NULL above can race with ep_remove_wait_queue()
-                * which can do another remove_wait_queue() after us, so we
-                * can't use __remove_wait_queue(). whead->lock is held by
-                * the caller.
-                */
-               list_del_init(&wait->task_list);
-       }
-
        spin_lock_irqsave(&ep->lock, flags);
 
        /*
@@ -924,6 +919,23 @@ out_unlock:
        if (pwake)
                ep_poll_safewake(&ep->poll_wait);
 
+       if ((unsigned long)key & POLLFREE) {
+               /*
+                * If we race with ep_remove_wait_queue() it can miss
+                * ->whead = NULL and do another remove_wait_queue() after
+                * us, so we can't use __remove_wait_queue().
+                */
+               list_del_init(&wait->task_list);
+               /*
+                * ->whead != NULL protects us from the race with ep_free()
+                * or ep_remove(), ep_remove_wait_queue() takes whead->lock
+                * held by the caller. Once we nullify it, nothing protects
+                * ep/epi or even wait.
+                */
+               smp_mb();
+               ACCESS_ONCE(ep_pwq_from_wait(wait)->whead) = NULL;
+       }
+
        return 1;
 }